1. Technical Field
This invention relates to the technology of eliminating chatter in the boring of metals, and more particularly to avoiding chatter resulting from the inter-action of several cutting elements on a single tool that produce different cutting forces in the metal chip removal process, the cutting elements each being at a different cutting diameter and depth, often referred to as plunge cutting.
2. Description Of The Prior Art
Chatter in a boring operation can make it difficult or impossible to meet surface finish requirements, can generate excessive vibration, and can cause premature tool failure through edge chipping. Assuming the boring apparatus itself is properly balanced (there being little or no forced vibration resulting from spindle imbalance, gear drive irregularities, or vibrations from adjacent tools, pumps or motors), the problem to be overcome is that of self-excited vibrations resulting from interaction of structural dynamics of the machine tool and the forces generated in the chip removal process.
The conventional approach is to symmetrically space the cutters for dynamic balance, and reduce chatter by changing the cutting speed, depth of cut, or width of cut per cutting insert. This is often fruitless if it increases the cutting force in a direction in which the machine tool or work piece lacks stiffness. In multi-diameter and multi-depth cutting tools, this conventional approach often leads to productivity decrease.
Chatter is of particular concern to the automotive industry when machining valve seats in an automotive cylinder head. There have been two attempts by the prior art to reduce chatter in "same-diameter" milling of valve seats using a plurality of cutters. In one, the cutting edges of a single insert are equally circumferentially spaced, but given a different cutting angle as a result of defining the edges about an eccentric axis (see U.S. Pat. No. 1,882,690). In the other, a large number of individual flat thin notched cutting blades are held in a head for same-diameter milling; the blades are spaced randomly irregularly about the rotating axis of the head. The randomness and irregular spacing promotes a smoother finish using notched blades so that they traverse randomly different cutting lines (see U.S. Pat. No. 2,036,656). Neither of these references provide a solution for eliminating chatter in plunge cutting using cutters at different diameters and depths.